This invention relates to charge pumps and other auxiliary pumps used on hydrostatic transmissions (xe2x80x9cHSTxe2x80x9d) for use in light duty applications. Such HSTs can either have their own housing and be attached to an axle driving apparatus, or can be incorporated within a housing that includes the components of the axle driving apparatus. A unit that contains an HST within the housing of an axle driving apparatus is often referred to as a integrated hydrostatic transaxle (xe2x80x9cIHTxe2x80x9d). The operation of such units are described in U.S. Pat. Nos. 5,201,692 and 5,314,387, the terms of which are incorporated by reference.
This invention presents a unique and novel manner of providing and mounting charge pumps and auxiliary pumps for both IHTs or stand-alone HSTs that are mounted to a separate transmission. As described in the ""692 patent, an HST generally comprises a pump that receives and is rotated by an input shaft driven by a vehicle engine. The pump includes a plurality of pistons that contact a swashplate to cause axial movement thereof when the pump rotates. The pump is hydraulically connected to a motor, which is similar in construction to the pump. The motor receives hydraulic fluid from the pump, and movement of the motor pistons against a swashplate causes rotation of the motor, which is connected to and drives an output shaft. The hydraulic connection between the pump and motor is a closed circuit; however, in any such circuit there will be deliberate and incidental leakage due to lubrication requirements, the high pressure of the hydraulic fluid and manufacturing tolerances. Thus, the HST requires a mechanism to replace fluid leaked from the closed circuit. This replacement fluid is commonly called make-up fluid.
In present HST designs, the pump and motor are often mounted on a center section that includes the hydraulic circuit therein. The hydraulic circuit includes two sides: a high pressure side and a low pressure side. The low pressure side is sometimes referred to as the vacuum side. These two sides are reversed when the vehicle motion is changed from forward to reverse.
Typically, the center section is mounted in a housing, and the housing provides a hydraulic fluid sump. Make-up fluid is brought from the sump into the low pressure side of the hydraulic circuit to replace fluid which is lost therefrom due to leakage. Specifically, check valves mounted directly into the center section or mounted in a separate plate that is in communication with the center section, as shown in the ""692 patent, provide a fluid flow path between the sump and the hydraulic circuit. However, this arrangement often does not provide sufficient fluid flow into the low pressure side of the circuit to replace the lost fluid. Therefore, a charge pump may be used to assist in this process. In addition, the use of an auxiliary pump to supply pressurized hydraulic fluid for various purposes is generally known. This invention provides for an efficient manner of mounting a charge pump and, in some cases, an auxiliary pump, external to the housing but still in direct communication with the hydraulic circuit in the center section.
The external charge pump confers significant benefits with respect to the accessibility on the hydraulic system design and configuration. The external nature of the charge pump allows direct access via a simple hydraulic fitting to hydraulic fluid that can be used for auxiliary functions. Internal charge pumps, in comparison, generally require a complex series of chambers, connections and fittings in order for fluid to be accessible exterior to the housing. External pumps provide accessibility without unit disassembly, thereby allowing replacement, addition or upgrade of a charge or auxiliary pump. Previous configurations were not accessible without disassembly of the HST or IHT.
Additional benefits and features of this invention will be disclosed in the description of the drawings.